feat(ui): shared LoadingState — a centered accent spinner instead of plain loading text

Replaces the plain "Loading…" text in every R3 loading branch with a single shared LoadingState
(a spinning lucide Loader2 in the accent colour + label, centered, role=status/aria-live) — one
component so all loads read the same and can be swapped for skeletons later in one place. Applied
across Feed, Channels, ChannelDiscovery, AuditLog, NotificationsPanel, Messages, Scheduler, Stats,
PlexBrowse (grid + subviews), PlexPlaylistView, ConfigPanel, DownloadCenter, AdminUsers, ChatThread.
Left the small "load more" footer and lazy Suspense fallbacks as-is.

frontend/src/components/QueryState.tsx

@@ -1,6 +1,23 @@
 import type { ReactNode } from "react";
 import { useTranslation } from "react-i18next";
-import { AlertTriangle } from "lucide-react";
+import { AlertTriangle, Loader2 } from "lucide-react";
+
+// Centered loading state: a spinning accent ring + an optional label. The single shared loading
+// visual for a page/section's first load, so all of them read the same (and can be swapped for
+// skeletons later in one place). `role=status`/`aria-live` announces it to screen readers.
+export function LoadingState({ label, className = "" }: { label?: ReactNode; className?: string }) {
+  const { t } = useTranslation();
+  return (
+    <div
+      className={`flex flex-col items-center gap-3 py-10 text-sm text-muted ${className}`}
+      role="status"
+      aria-live="polite"
+    >
+      <Loader2 className="w-6 h-6 animate-spin text-accent" aria-hidden />
+      <span>{label ?? t("common.loading")}</span>
+    </div>
+  );
+}
 
 // Centered inline status line for a page/section's loading and error(+retry) states, in the house
 // style (mirrors Playlists' error block). Text-only by design — R3's goal is to stop pages